Need to Rough in Shower Drain in Preexisting Joist Frame

I have brought my downstairs bathroom down to the stud frame walls and the joist frame of the floor after finding dry rot extending into subflooring, one joist, and the stud frame of the wall between the bathroom and the adjacent kitchen. Because of the rot in the stud frame, I have taken that one wall down in its entirety.

The original shower pan was an ancient piece made of ceramic. It stood about eight inches tall and was plumbed like a shallow tub, with the drain pipe running horizontally into a downward directed overflow pipe that tied into the shower drain. As such, the drain, constructed of welded 1 1/4" brass pipe, with the trap in place, is positioned very close to the wall on the plumbed side of the shower.

Set-up as it is, it would be easiest to replace the shower unit with a tub and not have to worry much about the position of the drain and vent or the joist frame. However, I intend to put in a shower. I would prefer using a premanufactured shower pan to constructing one for myself, although I would be comfortable doing the latter if it made more sense in the context of the questions I have here.

Taking other issues into account regarding the layout of the room, I am left to consider a 34" x 42-48" shower pan. Most of the available units have the drain centered in both dimensions. I have found also two 34" x 48" units by Kohler, of which one has the drain centered on the left end about eight inches from the end and the other has it centered toward the back also about eight inches from the back wall. Of course, if I were to construct a shower pan for myself I would have more freedom with the placement of the drain.

The issues are these: For any of the units with the drain centered in both dimensions and for that centered on the left end, the drain would be positioned so that the drain pipe's position would intersect with an existing joist. If I chose the unit with the drain centered near the back wall and left the joist frame as it is, the near edge of the drain would clear the adjacent joist by about 1 13/16". If I were to devise a shower pan, I imagine I could design it so the drain was positioned in the very middle of one of the joist spaces if I so chose.

These are my questions:

1. I have been unable to find any printed info on the topic of roughing in a shower drain in terms of the joist frame. The original drain was positioned in an opening involving three adjacent joists, with the middle of the three cut short and the three tied together with a transverse board butt-ended into the cut joist and toenailed into the two on either side, as if to make a box for the drain. I believe that was necessary because otherwise the middle joist would have been in the path of the drain. If I choose to use a shower pan that would require a similar treatment to make room for the drain, are there specific minimum clearances that must be considered?

2. If I elect to use the pan with the drain centered on the back wall or I devise my own pan, in either of which case the drain would not fall directly in the path of a joist, are there required clearances, anyway (as for example in the former case, is 1 13/16" adequate clearance from the adjacent joist, or does it need to be farther away)?

3. Should I be considering anything else in this context?

4. The drain and vent for this shower are situated in an area that is very busy with other water, waste, vent, and heating pipes, and I think I will be more comfortable hiring a professional plumber to reposition those pipes and the new drain than to try to do it myself. Will it work out better if I let the plumber know the layout and have him do his work first or if I make any necessary changes in the joist frame, put down the subflooring with the opening for the drain, and position the pan before I then call the plumber to fit the drain to the system as it is laid out?

I have attached a jpeg image of the joist frame with the intended stud frame in place to help flesh these questions out. I intend to sister the joists, which are only 2 x 8's, to stabilize the floor for tiling. This image started out as my plan for that, and I sketched in the stud frames for the wall to be rebuilt and the two ends of the shower enclosure, along with the site of the drains I have outlined.

If you can find a manufactured pan that meets your needs, they can put the drain where they want and still maintain the needed slope. If you build one, you can, too, but that can be problematic. You need 1/4" per foot from the corner to the drain on the longest dimmension. This means if the drain is not centered, that you need a much steeper slope on the shorter distance in order to maintain the proper slope on the longer end as it has to drop the same distance in a shorter length. If you're tiling it, it makes it harder to get a decent looking transition, and means using smaller tile is better.

Check out www.johnbridge.com for tiling help. I'd probably tile the pan I built, and put in a header so I could center the drain properly. ALso, keep in mind that today's code requires a 2" drain for a shower...an older one may have had a 1.5" drain, and you'll have to replace all of it back to where it gets to at least 2".

While you're looking around, check out www.schluter.com and watch the videos of their shower system - Kerdi. A neat way to make a totally waterproof (rather than water resistant) shower, from the floor to the showerhead.

Thanks. Yet more to think about! It's just wonderful finding yourself in the position of remodeling a bathroom constructed back in the 40's. In fact, it wasn't even part of the original house. They just decided to build it in a corner of a huge kitchen, so there are all sorts of marvels with the carpentry, the plumbing, and the electricity. You name it!

I decided to bite the bullet, order a prefabricated pan with a centered drain, and cut out an opening in the joist system to allow easy clearance of the drain. I am hopeful the issue of slope on the drain and vent will not be too much of an issue. By putting the drain in the same line as the initial drain relative to the joists, they should be following their initial course, just at a steeper slope, and if anything it will simply require notching out a block to allow placement of the vent pipe. That blocking I'm putting in to to help tie the subflooring down securely but it does not represent a true structural support surface.

I've included the diagram of the altered joist system that I have planned (actually, I tried to, but I'm not sure it went through!).